1. Field of the Invention
The present invention relates to digital voice communication; and, more particularly, to a digital voice communication system and method that involves the radio transmission of synthesized speech.
Although the present invention is suitable for many different voice communication systems that involve switching "on" and "off" of voice transmission during periods of silence, it is particularly advantageous for use in cellular digital telephone systems and is described in connection therewith.
2. Discussion of Related Art
A cellular communication system is a mobile telephone service where radio coverage is divided into cells; and each cell is assigned a number of available radio frequencies. A mobile telephone station transmits and receives control and voice communication information from a base station within the same cell. The base stations are controlled by a cellular system switching and control network that provides connection with the world wide telecommunication system.
In digital communication systems, assigned frequencies are divided into individual channels of communication, with the transmit and receive frequencies being separated from each other. Each channel of information has a frame format, that is, each channel transmits a succession of frames, which has a duration typically of forty milliseconds, and constitutes one cycle of a regularly recurring series. Each frame of information is transmitted in one of six time slots. Each slot includes one hundred sixty-two symbols, and has a duration of approximately 6.67 milliseconds. Each slot corresponds to a burst of RF energy that includes compressed digital speech signals, which are decompressed at the receiving station and converted to analog speech.
An encoder is provided in each transmitter, both at a base station and a mobile station, which synthesizes the speech signals before modulation and transmission thereof. One type of cellular communication system includes a technique for low rate speech coding, referred to as Codebook Excited Linear Prediction (CELP), which involves searching a table or codebook of randomly distributed excitation vectors for that vector which, when filtered through pitch and linear predictive coding short term synthesis filters, produces an output sequence which is closest to the input sequence. This output sequence of synthesized speech codes occurs upon excitation of the input sequence which, in turn, occurs upon the introduction of the digital equivalent of analog speech.
Upon the detection of voice inactivity, which occurs between words, sentences, or pauses in conversation, for example, the input to the encoder is switched off, which interrupts transmission of the RF energy. This switching on and off of the transmitter during a conversation produces audible switching artifacts, which at times leads the listener to believe the connection is being inadvertently interrupted, and at the very least, causes the listener substantial annoyance and discomfort.
Heretofore, it has been proposed to produce an artificial background noise during periods of voice inactivity. This was in the form of background noise that was encoded and generated independently of the conversation preceding the inactivity. Although suitable for the purposes intended, the proposed background noise generation was at times substantially different from the background noise of the conversation during periods of voice activity, which may be unpleasant and disconcerting to the listener.